Summary

This module provides for dynamic health checking of balancer
members (workers). This can be enabled on a worker-by-worker
basis. The health check is done independently of the
actual reverse proxy requests.

Parameters

The health check mechanism is enabled via the use of additional
BalancerMember parameters, which are configured in the standard
way via ProxyPass:

A new BalancerMember status state (flag) is defined via this module: "C".
When the worker is taken offline due to failures as determined by the health
check module, this flag is set, and can be seen (and modified) via the
balancer-manager.

Parameter

Default

Description

hcmethod

None

No dynamic health check performed. Choices are:

Method

Description

Note

None

No dynamic health checking done

TCP

Check that a socket to the backend can be created: e.g. "are you up"

OPTIONS

Send an HTTP OPTIONS request to the backend

*

HEAD

Send an HTTP HEAD request to the backend

*

GET

Send an HTTP GET request to the backend

*

*: Unless hcexpr is used, a 2xx or 3xx HTTP status will be interpreted as passing the health check

hcpasses

1

Number of successful health check tests before worker is re-enabled

hcfails

1

Number of failed health check tests before worker is disabled

hcinterval

30

Period of health checks in seconds (e.g. performed every 30 seconds)

hcuri

Additional URI to be appended to the worker URL for the health check.

hctemplate

Name of template, created via ProxyHCTemplate to use for setting health check parameters for this worker

hcexpr

Name of expression, created via ProxyHCExpr, used to check response headers for health.If not used, 2xx thru 3xx status codes imply success

In this scenario, http://www.example.com/ is health checked by sending a GET /status.php
request to that server and seeing that the returned page does not include the string Under maintenance. If
it does, that server is put in health-check fail mode, and disabled. This dynamic check is performed
every 30 seconds, which is the default.

http://www2.example.com/ is checked by sending a simple HEAD request every
10 seconds and making sure that the response status is 2xx, 3xx or 4xx. http://www3.example.com/ is checked
every 5 seconds by simply ensuring that the socket to that server is up. If the backend is marked as
"down" and it passes 2 health check, it will be re-enabled and added back into the load balancer.
It takes 3 back-to-back health check failures to disable the server and move it out
of rotation. Finally, http://www4.example.com/ is
not dynamically checked at all.

The ProxyHCExpr directive allows
for creating a named condition expression that checks the response
headers of the backend server to determine its health. This named
condition can then be assigned to balancer members via the hcexpr
parameter

If Apache httpd and APR are built with thread support, the health check
module will offload the work of the actual checking to a threadpool
associated with the Watchdog process, allowing for parallel checks.
The ProxyHCTPsize directive
determines the size of this threadpool. If set to 0, no threadpool
is used at all, resulting in serialized health checks. The default size is 16.

ProxyHCTPsize

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